Pneumatic control rod drive including a scram cushion

ABSTRACT

A control rod drive for a nuclear reactor includes a vertically disposed pneumatic cylinder containing a piston having shafts extending above and below the piston. The lower shaft is connected to the control rod and the upper shaft extends through a second, short pneumatic cylinder to terminate in an enlarged head. Air is supplied to the pneumatic cylinder below the piston during normal operation of the reactor to force the enlarged head of the upper shaft against a stop nut on a lead screw, the position of the stop nut on the lead screw determining the position of the control rod within the reactor. To scram the reactor high-pressure air is admitted to the pneumatic cylinder above the piston while the air below the piston is simultaneously exhausted. Deceleration of the control rod is obtained by admitting the high-pressure air used for scramming the reactor through a check valve into the second, short pneumatic cylinder below a free-floating piston therein. When the upper shaft head contacts the piston air is compressed in the second cylinder cushioning the fall of the control rod. A bleed line is provided by passing the check valve and connecting the short cylinder to the scram gasline.

nited States Patent William E. Taft [72] Inventor Los Gatos, Calif. [21]Appl. No. 866,928 [22] Filed Oct. 16, 1969 [45] Patented Oct. 19, 1971[73] Assignee The United Slates of America as represented by the UnitedStates Atomic Energy Commission [54] PNEUMATIC CONTROL ROD DRIVEINCLUDING A SCRAM CUSHION 2 Claims, 2 Drawing Figs.

[52] US. Cl 92/13.8, 92/8, 91/43 [51] Int.Cl F0lb3l/00 [50] Field ofSearch 92/8, 10, 85, 13.8; 176/36; 91/43 [56] References Cited UNITEDSTATES PATENTS 3,273,469 9/1966 Doyle 92/85 3,162,578 11/1964 Allen.....92/85 X 2,855,899 10/1958 Beaty 176/36 2,821,172 1/1958 Randall 91/43 XPrimary Examiner-Edgar W. Geoghegan Assistant Examiner-R. H. LazarusAtt0mey-Roland A. Anderson ABSTRACT: A control rod drive for a nuclearreactor includes a vertically disposed pneumatic cylinder containing apiston having shafts extending above and below the piston. The lowershaft is connected to the control rod and the upper shaft extendsthrough a second, short pneumatic cylinder to terminate in an enlargedhead. Air is supplied to the pneumatic cylinder below the piston duringnormal operation of the reactor to force the enlarged head of the uppershaft against a stop nut on a lead screw, the position of the stop nuton the lead screw determining the position of the control rod within thereactor To scram the reactor high-pressure air is admitted to thepneumatic cylinder above the piston while the air below the piston issimultaneously exhausted. Deceleration of the control rod is obtained byadmitting the high-pressure air used for scramming the reactor through acheck valve into the second, short pneumatic cylinder below afree-floating piston therein. When the upper shaft head contacts thepiston air is compressed in the second cylinder cushioning the fall ofthe control rod. A bleed line is provided by passing the check valve andconnecting the short cylinder to the scram gasline.

PATENTEnnm 191971 I Inventor Zllz'llrlam E. Taft flzorflqy PNEUMATICCONTROL ROD DRIVEINCLUDING A SCRAM CUSHION BACKGROUND OF THE INVENTION 7This invention relates to a control rod drive for a nuclear reactor. Inmore detail the invention relates to a control rod drive including anovel scram cushion.

Control rods containing a material of high neutron capture cross sectionare used in most nuclear reactors to adjust the power output of thereactor to a desired value. In many reactors the same control rods usedfor routine control of the reactor are used to scram the reactor ordrive the control rods into the reactor to shut down the reactor asrapidly as possible. Since control rods are quite heavy and must bemoved over a distance of many feet at very high speeds to be effectiveunder scram'conditions, damage to the-structure of the reactor and tothe control rod will occur. unless means are supplied for smoothlydecelerating'the rod as it approaches the proper position within thereactor.

SUMMARY OF THE INVENTION This invention is concerned with operatingmechanisms for control rods designed to operate in a vertical positionin which the force of gravity assists in driving the control rod intothe reactor. The invention has for its principal object the provision ofa simple and reliable control rod operating mechanism which is capableof positioning a heavy, elongated control rod within a nuclear reactorat any desired location, is capable of moving the control rod into thereactor in the minimum possible time and, in addition, is automaticallyoperable to smoothly decelerate and stop the rod after it reaches aneffective position within the reactor. This is accomplished by employinga lead screw and lock nut mechanism to position the control rod withinthe reactor, pneumatic means to drive the control rod into the reactorunder scram conditions and a cushioning device comprising a gascompression chamber filled with high-pressure scram gas wherein afree-floating piston operating against the gas slows down the controlrod near the end of its stroke. A bleed line is provided to preventbounce back of the control rod.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a partial vertical sectionof a control rod drive according to the present invention showing thedrive in normal operating position.

FIG. 2 is an enlarged vertical section of a portion thereof showing thedrive in scram position.

DESCRIPTION OF THE PREFERRED EMBODIMENT Reference is now made to theaccompanying drawing wherein the main element of the control rod driveis a vertically mounted pneumatic cylinder 10 enclosing a piston 11provided with an upper shaft 12 extending through an upper closuremember 13 and a lower shaftl4 extending through a lower closure member15. Lower shaft 14 extends into the reactor head 16 of a nuclear reactorand carries the load of a control rod (not shown). The control rod driveis mounted on reactor head 16 by means of a mounting member 17 which isspaced from the reactor head during normal operation of the reactor byspacer l8. Spacer 18 is a two-part split ring assembly which can beeasily removed when the weight of the control rod and drive mechanism isremoved from it. This is done to move the control-rod shaft down torelease the load on the coupling with the control rod. THis must bedonebefore the drive shaft and control rod are uncoupled.

Upper shaft 12 is hollow having an open end and extends up into acylindrical housing 19 mounted in line to the upper end of pneumaticcylinder 10 and separated therefrom by spacer 20. Housing 19 has opposedlongitudinal slots 21 therein extending down to a point somewhat abovethe bottom of housing 19. A lead screw 22 having a stop nut 23 mountedthereon may be rotated within housing 19 by motor 24. Lead screw 22 isinsertable into hollow shaft 12 as shown. Stop nut 23 has rollers 25mounted on opposite sides thereof which rollers ride within slots 21.Thus the stop nut may be positioned within housing 19 by rotating leadscrew 22. A collar 26 having a portion 27 of width slightly less thanthat of thehousing l9 and a lower portion 28 of much smaller diameter isscrewed over the top portion of the upper shaft 12 and is retainedthereon by lock nut 29. Portion 27 of collar 26 bears rollers 30 onopposite sides thereof which ride in slots 21. This structure preventsrotation of shaft 12 during operation of the reactor, This feature isnecessary since the drive shaft and control rod are uncoupled byrotating the drive shaft 45 (following disengagement of the antirotationdevice).

A source of high-pressure gas is connected to cylinder 10 by line 31which passes through upper head 13 of cylinder 10. To drive the controlrod rapidly into the reactor air or some other gas -is introducedthrough this line. During normal operation of the reactor air isintroduced into cylinder 10 through line 32 passing through lower head15 of cylinder 10. The pressure used is sufficient to raise piston 11 sothat collar 26 presses against stop nut 23. Thus the location of stopnut 13 determines the position of the control rod in the reactor. Duringa scram, when the control rod is being forced rapidly into the reactor,the gas below piston 11 is exhausted from the pneumatic cylinder 10.

As has been indicated it is essential that a cushion or bumper beprovided which will absorb the shock resulting from the rapid insertionof the control rod into the reactor. Scram cushion device 33accomplishes this result by decelerating the control rod from itsdownward scram motion during its last few inches of travel. Scramcushion device 33 consists of a piston 34, having a narrow extension34aon the upper side thereof, riding freely on shaft 12 within a shortpneumatic cylinder defined by the lower portion of housing 19, a wearring 35 on member 20 and an annular member 36 having an opening thereinlarger than the diameter of narrow extension 34a of piston 34 which isattached to housing 19 just below the bottom of slots 21. When a scramis initiated a portion of the high pressure scram gas is directed to thelower side of piston 34 through line 37 past check valve 38 which lineenters the short pneumatic cylinder through member 20 and water ring 35.This forces piston 34 upwardly against annular stop 36. As the controlrod is driven downwardly the narrow portion 28 of antirotation device 26contacts cylindrical extensions 34a of piston 34 which starts tocompress the gas in the short pneumatic cylinder and decelerate thescram motion. It will be observed that both extension 34a and portion 28are narrower than is the opening in annular stop member 36 so that theseelements are continuously in contact following initiation of thecushioning action. To prevent bounce back a properly sized bleed line 39is placed in parallel with check valve 38.

The scram cushion device as described is advantageous since highpressure is in the short pneumatic cylinder at the beginning of thecushion stroke of piston 34 so that not much piston travel is requiredbefore an effective cushioning action is created. Another advantage ascompared to conventional devices is the relative ease with which thesingle-bleed orifice can be sized to an accurate diameter to preventeither excessive impact or bounce back.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:

1. In a control rod drive for nuclear reactor including a pneumaticcylinder, a double-shaft piston slidably mounted in said cylinder, oneof said shafts being connected to a control rod, means for adjusting theposition of the control rod including a collar on the other shaft, alead screw and stop nut assembly and a source of gas which urges thecollar against the stop nut assembly during normal operation of thereactor, and a source of high-pressure gas connected to said pnuematiccylinder above said piston which is capable of driving the control rodrapidly into the reactor, the improvement comprising means fordecelerating the control rod comprising a piston having a portion ofsmall diameter riding freely on the said other shaft within a shortcylinder located in line with and between the said pneumatic cylinderand the lead screw and stop nut assembly, an annular closure memberhaving an opening therein larger than the portion of small diameter ofthe piston disposed at the top of said short cylinder as a stop for saidpiston, a line provided with a check valve for directing gas from saidsource of high-pressure gas into said short cylinder below said pistonto force the portion of small diameter of the piston against the collaron the other shaft and a bleed line in parallel to said first-mentionedline between said short cylinder and said source of high-pressure gas.

2. A control rod drive for a nuclear reactor comprising a pneumaticcylinder, a piston having shafts at top and bottom thereof slidablymounted in said cylinder, the lower shaft carrying the load of thecontrol rod, the upper shaft extending into a cylindrical housingseparated from said pneumatic cylinder by a spacing member, saidcylindrical housing having opposed slots therein extending a part of thelength thereof, a lead screw having a stop nut mounted thereon rotatablein said second cylinder, said stop nut being guided by opposed rollersriding in said opposed slots, an antirotation collar attached to the topof the shaft including a portion of wide diameter and a portion ofnarrow diameter, said collar containing in the wide portion thereof apair of opposed rollers adapted to ride in said slots, a source ofhigh-pressure gas attached to said pneumatic cylinder above said pistonand of low-pressure gas attached to said pneumatic cylinder below saidpiston, said lowpressure gas normally holding the antirotation collaragainst the stop nut, the high-pressure gas serving to drive the controlrod rapidly into the reactor under scram conditions as the lowpressuregas is exhausted, an annular closure member having an opening thereinlarger than that of the shaft located in the upper cylinder just belowthe bottom of the slots, a piston freely mounted on said shaft within ashort pneumatic cylinder defined by the lower portion of said housing,said piston including a narrow portion of size just sufiicient to passthrough the opening in said annular closure member and contact thenarrow portion of the antirotationcollar, a line provided with a checkvalve for directing gas from said source of high-pressure gas into saidshort cylinder below said piston and a bleed line bypassing said checkvalve.

1. In a control rod drive for nuclear reactor including a pneumaticcylinder, a double-shaft piston slidably mounted in said cylinder, oneof said shafts being connected to a control rod, means for adjusting theposition of the control rod including a collar on the other shaft, alead screw and stop nut assembly and a source of gas which urges thecollar against the stop nut assembly during normal operation of thereactor, and a source of high-pressure gas connected to said pnuematiccylinder above said piston which is capable of driving the control rodrapidly into the reactor, the improvement comprising means fordecelerating the control rod comprising a piston having a portion ofsmall diameter riding freely on the said other shaft within a shortcylinder located in line with and between the said pneumatic cylinderand the lead screw and stop nut assembly, an annular closure memberhaving an opening therein larger than the portion of small diameter ofthe piston disposed at the top of said short cylinder as a stop for saidpiston, a line provided with a check valve for directing gas from saidsource of highpressure gas into said short cylinder below said piston toforce the portion of small diameter of the piston against the collar onthe other shaft and a bleed line in parallel to said firstmentioned linebetween said short cylinder and said source of high-pressure gas.
 2. Acontrol rod drive for a nuclear reactor comprising a pneumatic cylinder,a piston having shafts at top and bottom thereof slidably mounted insaid cylinder, the lower shaft carrying the load of the control rod, theupper shaft extending into a cylindrical housing separated from saidpneumatic cylinder by a spacing member, said cylindrical housing havingopposed slots therein extending a part of the length thereof, a leadscrew having a stop nut mounted thereon rotatable in said secondcylinder, said stop nut being guided by opposed rollers riding in saidopposed slots, an antirotation collar attached to the top of the shaftincluding a portion of wide diameter and a portion of narrow diameter,said collar containing in the wide portion thereof a pair of opposedrollers adapted to ride in said slots, a source of high-pressure gasattached to said pneumatic cylinder above said piston and oflow-pressure gas attached to said pneumatic cylinder below said piston,said low-pressure gas normally holding the antirotation collar againstthe stop nut, the high-pressure gas serving to drive the control rodrapidly into the reactor under scram conditions as the low-pressure gasis exhausted, an annular closure member having an opening therein largerthan that of the shaft located in the upper cylinder just below thebottom of the slots, a piston freely mounted on said shaft within ashort pneumatic cylinder defined by the lower portion of said housing,said piston including a narrow portion of size just sufficient to passthrough the opening in said annular closure member and contact thenarrow portion of the antirotation collar, a line provided with a checkvalve for directing gas from said source of high-pressure gas into saidshort cylinder below said piston and a bleed line bypassing said checkvalve.